1. Field of the Invention
The present invention relates to an image reading apparatus, such as a copying machine, a printer or a laser beam printer, provided with original feeding means for automatically feeding an original to an original platen, and to an image forming apparatus provided with such an image reading apparatus.
2. Related Background Art
Conventional methods of feeding an original by means of a sheet-material transporting device and reading an image therefrom by means of an image forming apparatus such as a copying machine are divided into two major types. One method comprises the steps of exposing and reading a sheet original upon halting it at a predetermined position on a glass platen, while moving an optical system, and the other method comprises the steps of exposing and reading a sheet original by means of an optical system fixed in position, while transporting sheet originals one after another.
The former method is utilized in a recurrence type of original transporting apparatus disclosed in Japanese Patent Laid-Open No. 58-148150. This apparatus is disposed on the top of the main body of a copying machine, and sheet originals are stacked on a tray with their processing surfaces facing up. One sheet original is sequentially separated from the original stack at the lowermost position thereof at a time and transported onto a platen glass disposed on the main body of the copying machine. After the sheet original has been halted in a predetermined position, only an optical system is moved to expose the sheet original. After completion of the exposure, the sheet original is discharged from the original platen. After the leading edge of the original is securely captured by the first grip rollers on the downstream side, the next original is transported onto the platen glass. Replacement of originals on the platen glass is accomplished by the operation of causing the successive originals to pass each other.
However, this conventional arrangement has a number of disadvantages. For example, replacement of originals is performed by the operation of causing the exposed original and the next original to pass each other. Accordingly, transport of the next original cannot be performed before the leading edge of the exposed original is securely gripped by the first transporting means on the downstream side. As a result, the time required for the leading edge of the original to be securely gripped after the completion of the exposure is needed for the replacement of the originals in spite of a lost time. This makes it difficult to improve throughput to a great extent.
A conventional arrangement utilizing the latter method is disclosed in Japanese Patent Laid-Open Nos. 60-140364 and 61-32836. In this arrangement, a continuous copying operation is achieved by sequentially transporting a sheet original to an exposure position, halting the sheet original at a predetermined position, moving an optical system alone, replacing the original with the next original immediately after reading has reached the trailing edge of the original, and simultaneously returning the optical system to its home position. The continuous copying operation leads to the improved throughput of the copying machine. However, the above method has the following disadvantage. The distance traveled by the next original from the leading edge of the halt position to the reading start position need be made equal to the length of the original plus the distance between successive originals. As a result, original replacement requires a long time and the throughput of the copying machine deteriorates.
The aforesaid method has a critical problem. Since the same portion is continuously illuminated by the illumination lamp of the optical system, the temperature of the platen glass or the original transporting device disposed thereon increases. In particular, an arrangement of a high-throughput type requires a high intensity lamp, and the amount of heat increases.
In order to solve the above problem, Japanese Patent Laid-Open No. 60-178441 discloses a method in which each time a predetermined number of originals are exposed, the halt position of an optical system is shifted. This method, however, still has a number of problems. For example, each time the halt position of the optical system is shifted, the read start position of the leading edge of the original varies. Accordingly, a complicated control method for positioning is required. The time required for the optical system to move to a different halt position is consumed as a lost time, thus leading to a decrease in throughput.
This method has further problems. For example, if the throughput of the method is to be improved to a further extent, it is also possible to adopt a method of reducing the time of image reading merely by increasing the moving speed of the original. In such method, since a sheet original is held by the friction of belts, rollers or the like, as the moving speed increases, the amount of slip or variation in the speed of rotation of a motor may increase. As a result, an image may be blurred and a satisfactory image cannot be obtained. If these problems are to be improved, a motor or a driving system having torque or inertia greater than would be otherwise unnecessary must be prepared, so that the cost and size of the apparatus will increase.